1. Field of the Invention
This invention relates to a twist drill preferably for dry drilling, which drill has a central center web with a drill tip lying on the drill axis, and with the drill axis as the axis of symmetry both of the chip flutes and also of the primary cutting edges, namely those parts of the cutting edges, the chip faces of which lie in the vicinity of the chip flutes.
2. Background Information
In at least one embodiment according to the present invention, the chip flutes and the drill center web are therefore substantially no different from those of an altogether standard drill, the chip flutes and drill center web of which are realized so that they are substantially symmetrical to the drill axis. The axial symmetry also relates to those parts of the cutting edges, the chip faces of which lie in the vicinity of the chip flutes.
The elimination of the need to add a drilling fluid or coolant-lubricant during drilling significantly reduces costs for disposal as well as the other well-known expenses that are incurred as a result of the use of the drilling fluid. In general, therefore, attempts are made to manufacture drills using such materials and having such cutting geometries that dry drilling is possible. On the other hand, dry drilling causes problems, such as difficulties in chip removal, increased wear on the cutting edges and the circular lands, severe heating of the drill, and when the drill is extracted at the end of the drilling process, loud noises and flying sparks, or even the breaking of the drill. Cutting force measurements show that during the retraction of the drill from the hole, torques can occur that even exceed the torque required for drilling. An additional force component that occurs during the retraction of the drill is a negative thrust force which can even lead to the tearing of the drill.
The problems observed during the retraction of the drill after the drilling process has been completed are caused by the increased heating of the drill in the vicinity of the drill cutting edges that occurs during dry drilling. This increased heating of the drill during the drilling process leads to a continuous increase in the diameter of the drill and thus also of the hole, which increases as the drilling depth increases. This phenomenon can easily be determined mathematically as a function of the temperature of the drill in its cutting area. In addition to the simple heat of chip removal caused by the chip formation, a further heating results from the friction of the circular lands or secondary cutting edges on the wall of the hole. At this point, no further cutting occurs, but rather a scraping with negative chip angles. However, the diameter enlargement related to the heating also has a disadvantageous effect at this point. In short, the consequence is that at the end of the drilling process, the diameter of the drill and accordingly the diameter of the hole are greater than at the beginning of the drilling. When the drill is retracted, this results in a severe jamming in the hole. One consequence of this jamming is an even more severe heating of the tool, the resulting further expansion, the above-mentioned loud drill noises and the significant increase in torque when the drill is retracted from the hole.
In the known technical literature, there are repeated references to the fact that the problems described above during dry drilling can be avoided by cooling the drill by means of compressed air. Other remedies include a minimal lubrication or the use of special lubricant coatings.